1. Field of the Invention
The present invention relates to a light-emitting device including semiconductor light-emitting elements such as light-emitting diode elements (hereinafter referred to as LED elements) or the like, more specifically, to a light-emitting device in which a semiconductor light-emitting element is driven alternately.
2. Description of the Related Art
In recent years, an LED element has widely been used for a backlight of a color display, illumination or the like, because the LED element is a semi-conductor light-emitting element, it has a long service life and is miniaturized, has excellent driving characteristic and high light-emitting efficiency, and is capable of emitting bright emission color.
In particular, recently, there is widely adopted an AC drive type-light-emitting device in which an alternating-current power source (hereinafter referred to as AC power source) is used for color light emission by a combination of LED elements and a fluorescent resin, the number of the LED elements connected in series is changed depending on a voltage level (or current level) of the AC power source, temporally changed, that is, the voltage level of the AC power source is switched, and the LED elements are lighted.
However, because the voltage level of the AC power source in the AC drive light-emitting device changes depending on a time, LED elements having a threshold are not lighted while the AC power source is in a low voltage level, and the light emitting of the LED elements are initiated after a voltage level of the AC power source reaches a predetermined level. Consequently, a phenomenon in which the light emission of the LED elements is not performed occurs in a switching area of the AC power source. This results in flicker of light in the light-emitting device.
As method resolving the flicker, there has been proposed a structure in which a phosphorescent material is added to a fluorescent resin covering the LED elements, the flicker is prevented from occurring by covering a non-light-emitting portion of the LED elements by use of afterglow of phosphorescence excited by the light emitting of the LED elements (for example, see Japanese Patent Application Publication No. 2010-103522).
A conventional light-emitting device as disclosed in Japanese Patent Application Publication No. 2010-103522 is described hereinafter with reference to FIGS. 9 and 10.
In FIG. 9, the light-emitting device 100 includes a substrate 102, a first LED element group 103 and a second LED element group 104 which are mounted on the substrate 102, a first delay phosphor 105 covering the first LED element group 103, and a second delay phosphor 106 covering the second LED element group 104. Here, the delay phosphor is considered as a mixture of a fluorescent material and a phosphorescent material. The light-emitting device 100 further includes a reflection cup 109 in which a transparent resin 107 for protection is molded.
As shown in FIG. 10, the first LED element group 103 is formed by a plurality of LED elements 103a to 103i, and the second LED element group 104 is formed by a plurality of LED elements 104a to 104i. The plurality of LED elements 103a to 103i are connected in series, and the plurality of LED elements 104a to 104i are connected in series. The plurality of LED elements 103a to 103i and the plurality of LED elements 104a to 104i are connected to electrodes 102a and 102b which are provided on the substrate 102 and are electrically connected to an AC power source, thereby the plurality of LED elements 103a to 103i and the plurality of LED elements 104a to 104i are lighted.
It is possible to form a variety of light-emitting colors by selecting a kind of the first LED element group 103 and the second LED element group 104, and kinds of the first delay phosphor 105 and the second delay phosphor 106, and prevent flicker of light emitted from LED elements from occurring, which is one problem of an AC drive by afterglow of the first delay phosphor 105 and the second delay phosphor 106.
In the light-emitting device disclosed in Japanese Patent Application Publication No. 2010-103522, a plurality of semi-conductor light-emitting elements mounted on the substrate are divided into at least two groups and driven by an AC power source. In the light-emitting device, afterglow is formed by covering the first LED element group 103 and the second LED element group 104 by the first delay phosphor 105 and the second delay phosphor 106, respectively, and it is possible to prevent the flicker of LED elements caused by the AC drive from occurring.
However, in Japanese Patent Application Publication No. 2010-103522, preventing the flicker from occurring and diversity of light-emitting colors are disclosed, but there is no disclosure about a consideration relating to chromaticity adjustment based on an arrangement, drive method and so on of a phosphor and a phosphorescent body emitting light even after light receiving is stopped.